1. Field of the Invention
The present invention relates to a parallel mechanism for moving and positioning a spindle head or a work table of a machine tool with three degrees of freedom and a control method for the mechanism. In particularly, it relates to a three degree of freedom parallel mechanism and a control method for the mechanism which are suitable for the application to a milling machine for machining aircraft parts or components.
2. Discussion of the Related Art
As parallel mechanisms of this kind, there has been known one which is disclosed in U.S. Pat. No. 6,431,802 and shown in FIG. 10 of the present application. The known mechanism (hereafter referred to as first prior art) is designed to parallel drive a spindle head 12 by three carriages 3 which are driven respectively by three ball screws 5 in a Z-axis direction. The three carriages 3 and the spindle head 12 are jointed by three jointing rods (8, 9). Joint means between the carriages 3 and the jointing rods 8 are constituted by respective pins 10 perpendicular to the Z-axis, while joint means between the spindle head 12 and the carriages 3 are constituted by respective universal joints 11. This mechanism is advantageous because of being simple. However, the position and orientation of the spindle head 12 are detected and controlled at the locations of the three carriages 3 which are far away from the spindle head 12, whereby dimensional errors in machining the components such as the jointing rods 8, 9 and the like lead directly to errors in positioning. Further, another problem arises in that errors due to thermal deformation of the jointing rods 8, 9 and the like are unable to compensate. Motion control with these errors being left gives rise to a further problem that deterioration takes place in positioning error as well as in controllability (responsiveness) in positioning control. Furthermore, because of jointing by the pins 10, in other words, because the sum of the degrees of freedom at opposite ends of each jointing rod 8 or 9 is four and less than five, there arises a further problem that each jointing rod 8 or 9 suffers from a bending stress to weaken the mechanism in rigidity.
A mechanism shown in FIG. 11 is also disclosed in European Patent No. EP 1,245,349B. In this mechanism (hereafter referred to as second prior art), sliders 4 are respectively guided along three guide ways extending in a Z-axis direction, and a pair of arms 3 are jointed to each slider 4 so that six arms 3 support a head platform 5 corresponding to a spindle head. Spherical joints 6 and 7 are used for jointing each arm 3 with the head platform 5 and the associated slider 4. Because of using the spherical joints 6, 7, this apparatus does not have any bending stress acting on each arm 3 and thus, is high in rigidity. However, the position and orientation of the head platform 5 are detected and controlled at the locations of the sliders 4 which are far way from the head platform 5, whereby dimensional errors in machining the components such as the arms 3 and the like lead directly to errors in positioning, as is the case of the aforementioned first prior art. Another problem also arises in that it is unable to compensate for errors caused by the thermal deformation of the arms 3 and the like. Motion control with these errors being left gives rise to a further problem that deterioration takes place in positioning error as well as in controllability (responsiveness) in positioning control. An addition problem also arises in that a turn range for the head platform 5 has to be restricted to a small range for the purpose of obviating the interference of each pair of the adjacent arms 3 with the associated slider 4 and the associated guide way 2.
Another mechanism shown in FIG. 12 is disclosed in Japanese unexamined, published patent application No. 11-10575. In this mechanism (hereafter referred to as third prior art), a mast 1 for detecting the position and orientation of an end plate 6 is provided in addition to three links 7 and three ball screws 19, wherein the position and orientation of the mast 1 is detected in the neighborhood of a base plate 3. This mechanism is higher in position detection accuracy than the aforementioned two prior art mechanisms. However, because detections are made at a position away from the end plate 6, part dimensional errors in machining the mast 1 lead directly to errors in positioning, as is the case of the aforementioned two prior art mechanisms. Another problem also arises in that it is unable to compensate for errors caused by the thermal deformation of the mast 1 and the like. Motion control with these errors being left gives rise to a further problem that deterioration takes place in positioning error as well as in controllability (responsiveness) in positioning control. In the device, the detected positions of the mast 1 along three axes (I, θ, φ) are inversely converted for feedback to command values to be given to respective actuators. Nevertheless, nothing can be done for countermeasures in the case that kinematic parameters for the inverse conversion functions differ from those which should be for the actual machine, due to errors in machining machine components or due to thermal deformation of the machine components.
In Japanese unexamined, published patent application No. 2002-263979, the assignee of the present application proposed a parallel mechanism of the type that holds a traveling plate by six rods, wherein the calibration for kinematic parameters are performed from calculated position information and measured position information. However, the mechanism is not designed to directly detect the position of the traveling plate because detectors for the present positions which make a base of the measured position information are composed of six motor position detection encoders and six angle sensors for detecting rotational angles of the respective rods. The mechanism uses twelve detectors for position detection and estimates the kinematic parameters for a five degree of freedom traveling plate, wherein a problem arises in that a large load is imposed on the calculation processing. Further, since the present position is detected at a place which is away from the traveling plate and is used as feedback information in control, there arises another problem that the errors involved in the kinematic parameters result in compounding the positioning errors and in degrading the controllability (responsiveness) in positioning control. A further problem arises in that the twelve detection axes used therein make the mechanism complicated in construction.